The present invention relates to a cooling system for a high-temperature gasifier such as the slag bath generator shown in copending application Ser. No. 642,899, filed Dec. 22, 1975 and assigned to the assignee of the present application. In such a gasifier, coal or other fossile fuel feedstocks are gasified by the introduction of oxygen and water vapor. The composition of the raw gas emanating from the gasifier depends on that of the fuel feedstock, the operating conditions of the selected process and the gasification media employed for the supply of oxygen (e.g., air). The raw gas contains varying proportions of carbon monoxide, hydrogen, methane, carbon dioxide and non-dissociated water vapor and proportions of nitrogen which vary in accordance with the gasification medium. Sulfur, normally contained in the fuel feedstock, occurs in the gas mainly in the form of hydrogen sulfide as well as in the form of organic sulfur compounds. Satisfactory gasification of the fuel feedstock is frequently possible only if its admixtures are also gasified or are made to enter a specific chemical reaction. Some fuel feedstocks must, therefore, be gasified in high-temperature gasifiers. That is, they must be gasified at a high operating temperature. The high operating temperature may also be necessary to discharge liquid slag and to obtain thermal breakdown of heavy hydrocarbons.
A high-temperature gasifier can reach operating temperatures of between 1500.degree. and 2200.degree. C. Slag bath generators are particularly suitable as high-temperature gasifiers and are characterized by a simple mode of slag discharge. That is, the slag collects at the generator bottom in the molten state and then discharges through an overflow weir. The fluid level in the generator can be established as desired by means of the overflow weir.
Certain of the older gasifiers are operated at atmospheric pressure or with a slight positive pressure of about 0.2 bar. More recent generators, on the other hand, utilize a pressure of 20 bar or more in the gasification chamber. The output for a given gasifier surface cross section is multiplied by the pressure so that only such so-called high-pressure gasifiers are being used if large outputs are required.
Particularly intensive cooling is necessary to meet the high gasifier temperature and high gas pressure. The gasifier is, therefore, provided with walls having tubes extending therethrough and through which cooling water flows in a closed circuit. These tubes are covered with a coating of refractory ramming compound so that they are not directly exposed to the high operating temperatures. Since the tubes are smooth, the ramming compound will not readily adhere to the tubes. In prior art gasifiers, pins are welded to the tubes, for example in two rows on each tube, extending in the longitudinal direction thereof, so that oppositely-disposed rows of pins of two adjacent tubes are inclined at an angle toward each other. This provides sufficient retention for the ramming compound on the tubes, at least when the gasifier is started up.
During the starting-up procedure, the ramming compound surface nearest the interior of the gasifier reaches a temperature of between 600.degree. and 800.degree. C, depending upon the thickness of the ramming compound. The melting point of the slag is above this temperature, usually between 900.degree. and 1500.degree. C. The slag deposited on the ramming compound, therefore, solidifies and reinforces the thermal insulation so that no further slag solidifies after a specific slag coating thickness has been reached. Any slag which is then precipitated within the gasifier remains liquid and flows into the slag bath at the bottom of the generator where it flows out through the aforesaid weir.
The slag continues to adhere to the interior of the gasifier wall for as long as the latter is operated at a uniform operating temperature. If the temperature is reduced (i.e., when the gasifier is shut down), there is a risk that the slag coating will become partially detached due to contractions. The tubes of the cooling system will, therefore, be exposed at these places either because the slag coating adheres too strongly to the ramming compound or the ramming compound itself has been reduced due to slag diffusion through the slag coating.
Freely-exposed cooling tubes in a gasifier of this type give rise to extreme operating risks. Normally, under the protection of the ramming compound and/or the solidified coating, the cooling tubes are exposed to a heat flux density of 50,000 to 100,000 kcal/hm.sup.2, but the heat flux density without the protection is approximately 1,000,000 kcal/hm.sup.2. Such a high thermal loading very readily causes film boiling which occurs particularly in the region of the pins at the inner walls of the cooling tubes. An integral vapor film will, therefore, be formed on the internal wall of the cooling tube adjacent the pins. This vapor film has a thermal insulating effect and prevents adequate cooling of the cooling tube wall. If a cooling tube is destroyed by the high thermal loading due to insufficient cooling, and if cooling water is discharged into the gasification chamber, this will result in uncontrolled generation of steam and an increase of pressure on the gas side with well-known consequences. Special safety precautions must, therefore, be provided in high-temperature gasifiers, and this includes a maximum degree of cooling.
A distinction is made in cooling between pure water cooling which can be described as a single-phase flow and boiling cooling, referred to as a two-phase flow (i.e., water and steam). Plain water cooling is obtained with a simple water flow without the formation of any vapor. This form is employed in older slag bath generators operated at atmospheric pressure and with a cooling water flow rate of 0.5 to 2.5 meters per second. Cooling water under slight water pressure up to about 4 atmospheres absolute reaches a temperature of approximately 135.degree. C.
Boiling cooling, in contrast to pure water cooling, is characterized by much higher thermal transfer coefficients and is used in high pressure gasifiers in view of the aforementioned physical conditions and the resulting need for safety. However, this does not preclude film boiling in the cooling tubes. The risk of boiling is merely reduced by the higher thermal transfer coefficients. If vapor bubbles form on the inside of the cooling wall tube in point contact therewith to a greater or lesser extent, such steam bubbles will become detached from the water flow, which is turbulent more particularly due to boiling, and form a second flow phase in addition to the water. These conditions may alter if the cooling tube wall is overheated because of the risk of an integral film being formed again. Despite the fact that the risk of film boiling is merely reduced, this process is almost exclusively employed in the more recent high-pressure gasifiers.
In cases in which boiling cooling is not employed, the cooling walls are protected against overheating by brickwork of approximately 450 to 500 millimeters thickness. These generators can be used at operating temperatures of up to 1600.degree. C only and, despite the thick brickwork, provide adequate protection for cooling walls for only a relatively short period. The reason for this is the action of the slag and the high temperature of the brickwork. As in the case of the ramming compound, the brickwork is reduced by the slag; and this action is substantially accelerated at operating temperatures above 1600.degree. C and causes destruction of the brickwork in a short time.